Night visitors: A Study of the Hourly Distribution of Moth Activity at a Light Station

The moths that were attracted to the ultraviolet light that I used for my study were so beautiful I took numerous photos to record their “visits.” This is an Imperial Moth, one of my personal favorites.

One night when I was about 3 years old, I found a luna moth at our front porch light. I was absolutely fascinated by its delicate beauty. Since then, I have been intrigued by the mystery of light attraction in night-flying insects. In 2003 I examined how weather conditions affect insect activity at night. In 2004 I chose to look at how moon phases affect moth activity at a light station. During the summer of 2005 and again in 2006, I collected data to determine if the effect of weather conditions (humidity and temperature) is stronger than the effect of moon phases on insect activity at a light station. For my study in 2007, I decided to use two light stations, one with an ultraviolet light and one with the incandescent light I had used in previous studies. The data from both light stations was compared, and I proved that insects are attracted in greater numbers to ultraviolet light than to incandescent light.

However, every year I studied phototropism in night-flying insects, I wondered what results I would get if I observed the insects for the whole night. During the summer of 2008, I decided to explore the idea. Using the ultraviolet light, I set up a light station in my yard and counted the moths at the station at two-hour intervals for 13 nights. I believe the results of this study have strong implications for conservation, especially in areas that are near nature preserves or national parks. Since populations of large moths continue to decline, an understanding of when moths are most active could assist in their preservation and study.

Grant adding a specimen to his collection.

The question of the time of night when moths are most active has been a subject of research for several years. As early as 1935, C.B. Williams conducted a study for the Royal Society of London that examined the activity of certain nocturnal insects by using a light trap. He used a fractionating light trap for four years to study flight activity as it changed during the night. The duration of each period of light trapping varied depending on the time of sunset and sunrise. A timed device separated the insect catch into eight groups, with the fourth period always ending at midnight. Williams found that he caught the largest number of insects in the first period and the fewest in the seventh. However, the time of peak flight activity among nocturnal insects varies by species. Lepidoptera species flew to the light in the highest numbers in the second, fifth, and eighth periods (Williams).

According to Mogens C. Nielsen, M. Lansing, and Gary Dun, some moths do not even fly until after midnight. Several scientists have noted that a significant difference exists between the flight periods of species even within the same family. According to a study conducted in Russia by W.E. Wallner, the highest activity periods for gypsy moths, for instance, is between 11 p.m. and 1 a.m. The black arches fly between 3 a.m. and 5 a.m., and rosy gypsy moths between 1 a.m. and 3 a.m. A. Ambrus and Gy Csoka determined that there is a difference in the light-trap periods of the two sexes of pine moth. The males fly to the light late at night, but females fly to light earlier. V.B. Tshernishev conducted a study that revealed that moths, particularly the Noctuidae and brown chafer species, typically fly with a very discernible nocturnal peak.

The most recent study on record of the peak times of moth activity was conducted by L. Nowinsky. His study was conducted with a light trap at a farm of the Plant Protection Institute between August 1976 and July 1979. The trap was not operated nightly but periodically. It was in operation for 57 nights. The results of this study were used to determine the peak activity time for moths that are harmful to crops. The greatest number of total moths, however, was found during the midnight-to-1 a.m. time frame (Nowinsky).

Insects other than moths were attracted to the light stand.

Taking into consideration the studies I had read and my own personal experience, my hypothesis for the study I conducted was that the moths would be active in greater numbers at approximately 2:30 a.m. The materials I used for my study included an ultraviolet field light, set up against a white sheet, and data sheets to record the numbers of moths found at the station at 10:30 p.m., 12:30 a.m., 2:30 a.m., and 4:30 a.m. Additional information recorded included the temperature, the moon phase, and weather conditions. I also set up a camera on a tripod to record time-lapse images of the insects flying in to the collection station. I set my alarm clock to wake me at the designated times to count the moths. I collected data on 13 nights during the summer to make sure that the weather conditions and moon phases varied.

After all the data had been collected, I averaged all the moth counts for each of the four times during the night that I counted them. I then created a chart to compare the totals. I was interested to see that the 2:30 a.m. time slot had by far the largest number of moths. This confirmed my hypothesis that moths would be most active at that time of the night. I found that the numbers of moths increased up until 2:30 a.m. and then started dropping as dawn approached.

Night Visitors Hourly Chart.

Since I had worked in previous summers on the effect of temperature on insect activity at night, I decided to examine how the temperature at each count affected the number of moths. To organize this data for a chart, I grouped the temperatures in ranges of five degrees: 55º-59º, 60º-64º, 65º-69º degrees, 70º-74º, and 75º-80º. I then averaged the number of moths counted at each of those temperature ranges. I found that the highest number of moths were counted at the 65º-70º range. The 75º- 80º range ran a close second, but the number of moths dropped significantly in the 70º-74º range. I suspected this drop was probably because of a pesky bat that frequented my light station on several of the nights that had temperatures in that range. But discounting that variable, the data still supports my previous finding that moths are more active on nights with warmer temperatures.

Night Visitors Temperature Chart

Since my previous studies had examined the effects of moonlight on insect activity, I decided to examine the relationship between that variable and the data that I had collected. I grouped the moth counts into five groups for this chart: counts made with a full moon, a 3/4 moon, a half moon, a 1/4 moon, and no moonlight visible. I then averaged the moth counts in each group to create the chart. The results were very interesting. Almost twice as many moths were counted at times with no moonlight than during any of the other moon phases. This reaffirmed the conclusion of my previous study. Moths are most attracted to light stations during times when the moon is not visible.

Night Vistors Moon Phases Chart

Moth populations in the United States have dropped 30% since 1968, with the most severe declines in urban areas where light pollution is particularly extreme (Adams). An awareness of when moth activity at night is at its peak suggests that outdoor lighting should be turned off at earlier hours in order to interfere less with the moths’ natural behaviors. More efforts should be made to restrict the use of blue- or purple-colored lights on buildings, signage, and in street lights, since moths are particularly attracted to those colors. Sodium vapor lights would be less attractive to moths. Amber-colored light-emitting diode (LED) floodlights have been developed that produce very little heat and are less attractive to insects, turtles, and other wildlife.

The effect of increased or inappropriate outdoor lighting has other undesirable side effects on many insect populations. For example, many butterfly larvae emerge and feed only at night. Artificial lighting is likely to inhibit this behavior and expose the larvae to a higher rate of predation. Some insects also use light for signaling between the sexes, and light pollution adversely affects these creatures. This study suggests that more consideration should be given to the effects of urban lighting on moths. Furthermore, I believe this study also shows that when scientists are conducting collections or counts of moths at night, the process should be conducted up to and after 2:30 a.m. for the best results. I also believe a study such as mine would be best suited to a team of scientists who could take turns making the counts of insects.

Bibliography

Adams, James K. “Why Are Moths Attracted to Lights?” Dalton, Georgia: Dalton State College Press, 2000.

Williams, C.B. “The Times of Activity of Certain Nocturnal Insects, Chiefly Lepidoptera, as Indicated by a Light Trap.” Trans. Royal Society of London 83 (1935): 523-556.

More About This Resource...

This winning entry in the Museum's Young Naturalist Awards 2009 is from a Tennessee eighth grader. Grant used his previous observations and research as a jumping off point for this study of moth activity. His essay discusses:

details about his life-long interest in the topic and previous studies he had conducted;

how he used his experience and research studies to develop a hypothesis about when moth activity would peak during the night; and

the results of his test for this hypothesis and how they related to what he had previously studied and learned about moth behavior.